A huge market exists for disk drives for mass-market computing devices such as desktop computers and laptop computers, as well as small form factor (SFF) disk drives for use in mobile computing devices (e.g. personal digital assistants (PDAs), cell-phones, digital cameras, etc.). To be competitive, a disk drive should be relatively inexpensive and provide substantial capacity, rapid access to data, and reliable performance.
Disk drives typically employ a moveable head actuator to frequently access large amounts of data stored on a disk. One example of a disk drive is a hard disk drive. A conventional hard disk drive has a head disk assembly (“HDA”) including at least one magnetic disk (“disk”), a spindle motor for rapidly rotating the disk, and a head stack assembly (“HSA”) that includes a head gimbal assembly (HGA) with a moveable transducer head for reading and writing data. The HSA forms part of a servo control system that positions the moveable transducer head over a particular track on the disk to read or write information from and to that track, respectively.
Typically, a conventional hard disk drive includes a disk having a plurality of concentric tracks. Each surface of each disk conventionally contains a plurality of concentric data tracks angularly divided into a plurality of data sectors. In addition, special servo information may be provided on each disk to determine the position of the moveable transducer head.
The most popular form of servo is called “embedded servo” wherein the servo information is written in a plurality of servo sectors that are angularly spaced from one another and are interspersed between data sectors around each track of each disk.
Each servo sector typically includes at least a phase locked loop (PLL) field, a servo address mark (SAM) field, a track identification (TKID) field, a sector ID field having a sector ID number to identify the sector, and a group of servo bursts (e.g. an alternating pattern of magnetic transitions) that the servo control system of the disk drive samples to align the moveable transducer head with or relative to a particular track. Typically, the servo control system moves the transducer head toward a desired track during a “seek” mode using the TKID field as a control input. Once the moveable transducer head is generally over the desired track, the servo control system uses the servo bursts to keep the moveable transducer head over that track in a “track follow” mode.
In order to perform these seeking and tracking operations by the servo controller, a servo field sync-up operation needs to be performed by the read/write channel to detect a servo sector. For example, when a disk drive is powered-up, the head of the disk drive attempts to detect a servo sector as part of a servo field sync-up operation in which a SAM search loop is performed.
In this method, the head attempts to detect a servo preamble, such as a phase locked loop (PLL) field having a known 2T pattern, which allows for the read/write channel to recover the timing and gain of the written servo sector. Once a servo sector is identified, the signal timing and gain information can be obtained from the known PLL pattern and the servo sector fields can be detected.
In order to accomplish this, a SAM search loop is implemented by the read/write channel once a preamble is identified. Unfortunately, the SAM search loop may be implemented due to an incorrectly identified preamble. Because the SAM search loop is turned-on for a fixed time duration, when the SAM search loop duration expires, there may not be enough time to re-trigger the next SAM search loop to detect a SAM. There is therefore a need for a method and a disk drive that addresses these limitations.